Conventionally, as an IC card and an identification tag using RFID technology (these are hereinafter referred to collectively as “IC tag” as well), devices are known in which an IC chip having recorded information therein and a resonant capacitor are electrically connected to an antenna coil. In these devices, it is configured to implement identification or monitoring by transmitting an electric wave of a predetermined frequency from a transmission/reception antenna of a reader/writer to the antenna coil, to activate an IC tag, and then to read the information stored in the IC chip in accordance with a read command based on data communication via the electric wave, or depending on whether or not resonance occurs with the electric wave of the specific frequency. Additionally, many of IC cards are configured to be able to update the information read, and write history information.
As a conventional antenna module for use mainly as an identification tag, a device is available in which a magnetic core member is inserted into an antenna coil formed by spirally winding wire in a plane, so as to be substantially parallel to the plane of this antenna coil (see Japanese Patent Application Publication No. 2000-48152). The magnetic core member in this antenna module is made of an amorphous sheet or an electromagnetic steel strip. The magnetic core member is inserted into the antenna coil so as to be substantially parallel to the plane of the antenna coil, for reducing the thickness of the entire antenna module.
However, since the antenna module having the above-mentioned configuration has its magnetic core member made of an amorphous sheet or an electromagnetic steel strip, a Q factor usable for frequencies of about 100 kHz is obtainable, but for a case of a high frequency such as several MHz to several tens of MHz, there has been an inconvenience that its Q factor drops due to eddy currents occurring in the amorphous sheet or the electromagnetic steel strip of the magnetic core member. Particularly, in recent years, IC tags using the RFID technology operating at 13.56 MHz have come into practice, and the antenna module disclosed in Japanese Patent Application Publication No. 2000-48152 is not usable for tags operating in such high-frequency electric waves.
On the other hand, a sintered ferrite has hitherto been known as a magnetic core member usable at high frequencies. However, the sintered ferrite is comparatively fragile. Particularly, when a sintered ferrite plate is made thin for use as a magnetic core member in order to obtain a thin antenna coil, the magnetic core member is susceptible to breakage, thus imposing a handling quality problem that its actual usable environment is limited. In order to overcome this problem, an antenna coil has been proposed which has a relatively high rigidity and is designed to be usable at relatively high frequencies, by forming a magnetic core member of a composite material including soft magnetic metal, amorphous or ferrite powder or flakes, and a plastic or rubber (see Japanese Patent Application Publication No. 2002-325013).
Furthermore, Japanese Patent Application Publication No. 2000-113142 discloses an antenna module having a configuration in which an antenna coil is formed by spirally winding wire within a plane, and a flat-shaped magnetic core member is laminated thereon so as to be parallel to this antenna coil.
Furthermore, Japanese Patent Application Publication No. Hei 11-74140 discloses a dust core manufacturing method in which a metal powder, which is used as a choke coil magnetic core and which is made of a composite material, is aligned in a direction of extrusion during extrusion molding. Japanese Patent Application Publication No. 2002-289414 discloses a configuration using a composite magnetic body in which a flat metal powder is pressed onto an electric wave absorber for adhesion to the back or the like of a liquid crystal of a portable information terminal in order to satisfy a noise standard for 100-400 MHz.
By the way, in recent years, a reliable operating environment is needed for RFID-based IC tags operating at 13.56 MHz. For example, also in terms of their communication characteristics, the longest possible communication distance, as well as a flat, wide communication area for a reader/writer facing a tag are called for.
For example, in a case where an article to be identified is made of a metal, an antenna coil used as an identification tag has an electrically insulating spacer interposed between the antenna coil and the article in order to avoid influence by the article, and the spacer may be substituted for by a magnetic core member in some cases (see Japanese Patent Application Publication No. 2000-113142).
On the other hand, since there may be cases where an antenna coil is incorporated into various communication equipment, the antenna coil may be easily affected by a metallic part, even if it is not the article to be identified, positioned therearound. In order to avoid this, there is a device in which a shield plate is adhered to the back (surface for adhesion) of a communication surface to suppress fluctuations in communication characteristics caused by a metallic body (see Japanese Patent Application Publication No. 2002-325013).
Although the fluctuations in the communication characteristics can be prevented by the shield plate, this also means that the communication characteristics of the antenna coil are degraded to a certain level by the shield plate. Thus, from the viewpoint of enhancing the communication characteristics, the presence of the shield plate could be a serious negative factor.
To overcome this situation, in order to suppress degradation of the communication characteristics of an antenna coil due to the influence of surrounding metals, if an antenna module is configured such that the above-mentioned magnetic core member is interposed between the antenna coil and the shield plate, the shield plate can be made to function as if it were not present as viewed from the side of the antenna coil (Japanese Patent Application No. 2003-092893).
In an antenna module having a laminated structure including an antenna coil, a magnetic core member, and a shield plate, the magnetic core member in the middle performs both a function of allowing the antenna coil to exhibit its communication performance, and an electromagnetic shielding function of preventing the antenna coil from being affected by the shield plate.
However, magnetic properties required of the magnetic core member to allow the antenna coil to exhibit the required communication performance are not necessarily compatible with magnetic properties required of the magnetic core member to satisfy the electromagnetic shielding function between the antenna coil and the shield plate. Thus, the present situation calls for proper selection of a magnetic core member that can trade off the communication characteristics of the antenna coil against its electromagnetic shielding function from the shield plate.
The present invention has been made in view of the above circumstances, and therefore has an object to provide a magnetic core member, an antenna module, and a portable communication terminal having this, which has a configuration capable of satisfying enhancement of the communication characteristics of the antenna coil, as well as its electromagnetic shielding function from the shield plate.